1. Field of the Invention
The present invention relates to a system for measuring airborne particles in a gas under vacuum, and a method for carrying out this measurement.
2. Description of the Prior Art
Many industrial processes, particularly those used in the microelectronic industry for fabricating integrated circuits are carried out in process chambers operated under conditions of reduced pressure. The processing not only is carried out in vacuum chambers, but must be done under extremely clean conditions because small particles suspended in a gaseous environment or atmosphere in the process chamber can deposit on the wafer or chip being processed, causing the product to degrade in performance. Sometimes the products are actually made non-functional by very small particles that are deposited during the process being carried out. This results in a loss of yield of the product, and considerable economic loss to the manufacturer.
The processes occur in large process vacuum chambers, and involve pumping down the gas for processing and venting the chamber at the completion of a process. This cycling of the process chamber gas may result in the generation of large quantities of airborne particles. At the present very little is known about particle generation in vacuum systems because there is a lack of suitable instrumentation for directly making particle concentration measurements in vacuums. It is known that some of the particles will deposit on walls of the vacuum system or chamber, and then will be dislodged during interchange of the vacuum atmosphere in a process cycle. Particles also can be produced by homogeneous and heterogeneous nucleation, followed by vapor condensation.
A recent development in the area of particle measurement in vacuums is a device called the PM-100 Particle Flux Monitor, manufactured by the High Yield Technology Company of Mountain View, California. This device has been described in an article by Borden et al., entitled "Monitoring Particles in Vacuum-Processing Equipment" Microcontamination, 5:30-34 (1987). The undersigned inventor also has been involved in an evaluation of this type of device, as set forth in an article that has been submitted for publication in Aerosol Science Technology.
Another way that particles have been measured in vacuum systems is to place a test surface (generally a wafer that is being processed) in the process vacuum chamber. Particles deposited on the surface of the wafer can be detected and measured by a light microscope or photographic methods. The lower size limit that can be measured by this method is about two microns. Another method or system involves scanning a wafer surface using a laser beam and measuring the scattered light caused by the particles with an optical detection system. The equipment for making the particle measurement is extremely expensive, and even the best equipment is limited to a lower particle size detection limit of about 0.2 microns. The equipment does not count particles in the actual vacuum environment, that is, while the particles are suspended in the gas or atmosphere, but is usable only after the particles have been deposited on a test wafer surface, and the wafer has been removed from the process vacuum chamber.
Thus, the present methods of measuring particles or aerosols in a vacuum do not provide measurements substantially in real time, or very close thereto. In real time measurements, the generation and decay of particles in the vacuum can be followed without disturbing the ongoing process in the vacuum, that is, without breaking the vacuum or disrupting the manufacturing process.